1. Field of the Invention
The present invention relates to a homogenizer. The homogenizer may form part of an illumination system of a lithographic apparatus. The present invention also relates to a beam delivery system suitable for delivering radiation from a radiation source to a lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Conventional lithographic apparatus include an illumination system which may be used to modify properties of a radiation beam before the radiation beam is incident upon the patterning device. The illumination system may for example be used to adjust the outer radial extent of the radiation beam in a pupil plane, and/or in the case of an annular illumination mode to modify the inner radial extent of the radiation beam in a pupil plane. In some lithographic apparatus it is possible to switch between different forms of illumination mode such as annular, disc shaped, dipole and quadrupole. This may be achieved by moving different optical elements into and out of the radiation beam, each optical element applying a different illumination mode to the radiation beam. Recently, it has been proposed that a lithographic apparatus may use an array of individually controllable mirrors to obtain different illumination modes. Using an array of minors in this way has the advantage that illumination modes having a wide variety of different shapes may be formed.
Where an array of individually controllable mirrors is used to obtain the illumination mode, homogenizing optics may be used to homogenize the radiation beam before it is incident upon the array of individually controllable mirrors. A problem associated with the use of a homogenizer in this way is that the peak intensity of the radiation beam in some parts of the homogenizer may be so high that those parts of the homogenizer are damaged.